1) Field of the Invention
The present invention is a system and method of use to improve the performance of sonobuoys and other acoustic sensors that can be dropped and then can properly operate upon entry into the ocean. This improved performance can also be used with acoustic sensors that are periodically lowered into the ocean in which the sensors are used to find underwater targets. The method comprises coating such devices with superhydrophilic coatings. The coating allows acoustic surfaces to “wet” completely and immediately upon immersion; thereby, preventing or minimizing the formation of air bubbles on the surface of a device with acoustic sensors. The suppression of bubble formation allows the acoustic sensors to immediately generate high quality acoustic data.
2) Description of Prior Art
Air cavities/bubbles are detrimental to acoustic sensors because the cavities/bubbles scatter and reflect acoustic waves. As shown in FIG. 1, an acoustic sensor 50 is set in an ocean environment 10. In the figure, the acoustic sensor 50 has a rubber coating 60 where cavities or bubbles 80 form upon immersion. Thus, a typical sensor or an air-dropped device like a sonobuoy cannot operate effectively until the air cavities/bubbles surrounding the device have significantly dissipated.
There are varying ways to make coatings that exhibit superhydrophilicity water/air contact angles in which the angles are essentially zero. Superhydrophilic surfaces exhibit perfect wetting when the surfaces are immersed such that essentially no air bubbles will form on such surfaces when the surfaces are immersed. If drops of water are placed on a surface, the drops will immediately flatten out to coat the surface evenly with water. This effect is the exact opposite of superhydrophobicity.
Superhydrophobicity has the requirement of a hydrophobic surface with a special geometry/roughness that forms a very thin layer of trapped air on the surface. This is opposite of what is preferred because air layers are very good sound reflectors/scatterors. Instead, it is preferred to have a superhydrophilic surface in which the water is in intimate contact with the surface so that there are no trapped air bubbles to interfere with the acoustic sensing. To make a superhydrophilic surface, the surface roughness is not critical; but to make a superhydrophobic surface then the correct degree of surface roughness is crucial.
The known art for coatings used on air-dropped/deployed acoustic sensors (e.g., sonobuoys) is to coat such devices with an acoustically-transparent elastomer that is neither superhydrophilic nor superhydrophobic. Because of this coating, as soon as the device makes contact with the water, air bubbles form both on the surface of the device and in the vicinity of the device (due to the “splash” of entry).
The air bubbles get trapped by irregularities on the surface and on surface materials that do not wet well. Traditionally, a wash of detergent is applied to the rubber face of an acoustic sensor to minimize these irregularities by cleaning the surface and by lowering surface tension. The air bubbles are undesirable for acoustic sensing—mainly because the bubbles reflect and scatter acoustic waves. Thus, for a period of time after the sensor enters the water, the acoustic sensor is unable to function properly.
The reason for improper functioning is that the sensor cannot obtain some acoustic signals until the cloud of air bubbles that formed dissipates. In addition, air bubbles formed during water entry may persist and cling to the sensor surface for a period of time. This formation further interferes with the reception of the acoustic signals.
Normally, good wetting results from high surface energy surfaces. The energy of the surface helps drops of water spread out into a thin film on the surface. A low energy surface tends to make the water drops bead up because the surface tension of the water exceeds the surface energy and the water molecules can draw themselves into three-dimensional droplets. The surface energy of a surface depends chemistry (what atoms/molecules are exposed at the surface) and somewhat on the fine-scale structure of the surface. Of course, if there are low surface energy contaminants present on the surface; there will be incomplete wetting. Clearly, it is desirable for air-dropped sensors to generate an air bubble cloud that is as small as possible and to optimally prevent air bubbles from forming on and/or clinging to acoustic surfaces.
Superhydrophilic coatings provide an ideal means for addressing these problems. Primarily, a superhydrophilic coated acoustic window/surface will wet completely and immediately upon entry into the water. Bubbles will not form on this surface, nor will air bubbles form elsewhere to attach and stick to such surfaces. Thus, an acoustic sensor coated with superhydrophilic compounds will be able to work immediately upon entry into the water; thereby, minimizing the chances of losing contact with a target while waiting for air bubbles to dissipate.
In regard to the prior art, the Capron reference (United States Publication Serial No. 2008/0199657) does not address superhydrophilicity and instead the section gives a well-known definition for superhydrophobicity. Superhydrophobicity includes the requirement of a hydrophobic surface with a special geometry/roughness that forms a very thin layer of trapped air right on the surface. This is the exact opposite of what is the intent of the invention described below because air layers are too good as sound reflectors/scatterers. This definition implies that the liquid matches the roughness of the surface.
In general, superhydrophilic surfaces have a high surface energy that pulls a liquid drop flat onto the surface. The chemistry of the surface is important for this effect; yet, that is not mentioned in the cited reference. Furthermore, paragraph [0026] of the Capron reference describes some surface chemistries that could be used to make a superhydrophilic surface.
Paragraph [0030] of the Capron reference simply indicates that the superhydrophobic or superhydrophilic materials will be deposited as a coating on another surface. Paragraph [0057] includes a description of what kind of contact angle would be expected from a water drop on a superhydrophilic surface. This concept is known in the art.
The reference further discloses a product having superhydrophilic or superhydrophobic surface having physical properties with a substrate coated on a surface with a structuring layer added on to the surface and with a film deposited on the layer. The film is continuous, and the physical properties of the surface are conferred by the nature of the film, the surface of the layer and with the deposited film having roughness with nanometric-size dimensions. This is how the cited reference makes superhydrophobic or superhydrophilic surfaces. The Capron reference only details the surface roughness of the coating/film—the surface roughness is more important for superhydrophobic surfaces. One does not need to focus on surface roughness to make a superhydrophilic surface.